Disc drive memory systems are widely used throughout the world today. These systems are used by computers and devices including digital cameras, digital video recorders, laser printers, photo copiers and personal music players. Disc drive memory systems store digital information that is recorded on concentric tracks of a magnetic disc medium. Several discs are rotatably mounted on a spindle, and the information, which can be stored in the form of magnetic transitions within the discs, is accessed using read/write heads or transducers. The read/write heads are located on a pivoting arm that moves radially over the surface of the disc. The discs are rotated at high speeds during operation using an electric motor located inside a hub or below the discs. Magnets on the hub interact with a stator to cause rotation of the hub relative to the shaft. One type of motor is known as an in-hub or in-spindle motor, which typically has a spindle mounted by means of a bearing system to a motor shaft disposed in the center of the hub. The bearings permit rotational movement between the shaft and the hub, while maintaining alignment of the spindle to the shaft. The read/write heads must be accurately aligned with the storage tracks on the disc to ensure the proper reading and writing of information.
Spindle motors have in the past used conventional ball bearings between the hub and the shaft. However, the demand for increased storage capacity and smaller disc drives has led to the read/write head being placed increasingly close to the disc surface. The close proximity requires that the disc rotate substantially in a single plane. A slight wobble or run-out in disc rotation can cause the disc to strike the read/write head, possibly damaging the disc drive and resulting in loss of data. Further, resistance to mechanical shock and vibration is poor in the case of ball bearings, because of low damping. Because this rotational accuracy cannot be achieved using ball bearings, disc drives currently utilize a spindle motor having fluid dynamic bearings on the shaft and a thrust plate to support a hub and the disc for rotation. One alternative bearing design is a hydrodynamic bearing.
In a hydrodynamic bearing, a lubricating fluid such as gas or liquid or air provides a bearing surface between a fixed member and a rotating member of the disc drive. Dynamic pressure-generating grooves formed on a surface of the fixed member or the rotating member generate a localized area of high pressure and provide a transport mechanism for fluid or air to more evenly distribute fluid pressure within the bearing and between the rotating surfaces, enabling the spindle to rotate with more accuracy. However, hydrodynamic bearings suffer from disadvantages, including a low stiffness-to-power ratio and increased sensitivity of the bearing to external loads or mechanical shock events.
To increase stiffness, spindle motors have been attached to both the base and the top cover of the disc drive housing. However, in order to use top cover attachment, the motor is open on both ends, which increases the risk of oil leakage. This leakage among other things is caused by differences in net flow rate created by differing pumping grooves in the bearing. If the flow rates within the bearing are not carefully balanced, a net pressure rise toward one or both ends may force fluid out through a seal. Balancing the flow rates is difficult because the flow rates created by the pumping grooves are a function of the gaps defined in the hydrodynamic bearing, and the gaps, in turn, are a function of parts tolerances. Proper sealing is also critical. Bearing fluids give off vaporous components that could diffuse into a disc chamber. This vapor can transport particles such as material abraded from bearings or other components. These particles can deposit on the read/write heads and the surfaces of the discs, causing damage to the discs and the read/write heads as they pass over the discs.
Efforts have been made to address these problems. One design is a top-cover-attach conical bearing having two independent flow paths. This design uses asymmetric sealing and includes a centrifugal seal and a grooved pumping seal. Another existing design, the exclusion seal (x-seal), is used to seal interfacial spaces between the hub and shaft (shown in FIG. 4). The x-seal includes an asymmetric sealing design with a single thrust plate, wherein one end is pumped inward with thrust spiral grooves and the other end with a groove pumping seal. At the thrust bearing end, a centrifugal seal maintains oil level change in the capillary reservoir during static to dynamic stage, and non-operating shock. Tests have shown, however, that the centrifugal seal fails at about 500 G shock, and oil leaks through fill holes at about 500 G shock.
Mobile applications require higher non-operating shock than desktop or enterprise products. Laptop computers can be subjected to large magnitudes of mechanical shock as a result of handling. It has become essential in the industry to require disc drives to be able to withstand substantial mechanical shock. A sufficient sealing system that can withstand 1000 Gs shock is needed for mobile applications. Further, a need exists to increase shaft stiffness and dynamic parallelism (alignment of the disc surfaces to the plane of the actuator arm motion) while simultaneously lowering bearing power.